For the basis to study the testicular functions using dd-mice, the cycle of the seminiferous epithelium and stages in spermatogenesis, classification of late spermatogenesis, and postnatal development of spermatogenesis were examined by light and electron microscopy in the testes from these animals at day of birth and 1, 2, 3, 4, 5, and 8 weeks of age. For light microscopy, paraffin sections of the testes were stained with periodic-acid and Schiff reagent (PAS) and hematoxylin. In spermiogenesis, 16 steps of spermatids were distinguished according to shapes of the nuclei and acrosomes. Step 15 and 16 spermatids, which have similar nuclei and acrosomes, were easily distinguishable by positions of nuclei and appearance of basophilic granules in the cytoplasm; steps 15 and 16 were thus subdivided into 3 and 2, respectively. The granules were accumulations of ribosome-like particles. Step 13 spermatids were present with step 1 spermatids, step 14 with steps 2 and 3, step 15 with steps 4-6, and step 16 with steps 7 and 8. Thus, the cycle of the seminiferous epithelium was divided into 12 stages (stages I-XII) in the order of steps 1 to 12. Type A spermatogonia were present in all stages, and intermediate and type B spermatogonia appeared from stages II and VI, respectively. In primary spermatocytes, the resting phase appeared from stage VII, leptotene from stage VIII, zygotene from stage X, pachytene from stage III, diplotene in stage XI, and diakinesis and secondary spermatocytes in stage XII. The seminiferous tubules were lined by Sertoli cells and contained gonocytes at day of birth, type A spermatogonia appeared at 1 week of age, spermatogenesis proceeded until pachytene spermatocytes at 2 weeks, until step 1 spermatids at 3 weeks, and all stages of the seminiferous epithelium were completed at 5 weeks. The findings indicate that combination of germ cells in each stage of the seminiferous epithelium in dd-mice does not correspond to generally accepted one, step 15 and 16 spermatids are accurately classified by referring the position of nuclei and appearance of granules in the cytoplasm, and spermatogenesis in immature mice proceeds faster than in adults.